A typical photovoltaic module comprises a plurality of solar cells that are connected in series by means of metal connectors. The solar cells are laminated into an embedding material which is isolating and serves for protection against weather conditions. Nowadays system voltages of several 100 V are regularly generated by series connection of the solar cells in a photovoltaic module and by series connection of several such photovoltaic modules to a system, which result in very high electric fields between solar cells and ground potential that result in undesired shift currents and leakage currents via the series circuit. As a result particularly charges may be deposited on the surface of the solar cells persistently that can significantly reduce the parallel resistance and thus their efficiency. In photovoltaic modules this process is particularly possible and maintained, because the solar cells are not encapsulated and isolated within the module so hermetically as this should be possible due to the material properties. The encapsulant material, in which the solar cells are embedded, the module frame and even the front cover glass allow the formation of leakage currents. These do not only exist in crystalline silicon solar cells but also in thin-film solar cells.
Hold J. Berg, O. Frank et al., “Potential Induced Degradation of solar cells and panels”, 5th World Conference on Photovoltaic Energy Conversion, 6-10 Sep. 2010, Valencia, Spain, pp. 3753-3759, and S. Pingel, O. Frank et al., “Potential Induced Degradation of solar cells and panels”, Proceedings of the 35th IEEE PVSC, 2010 disclose a procedure for testing modules, wherein a constant and continuous film of water is applied on the front cover glass by spraying or wherein a high humidity of e.g. 85% is set and wherein a high-voltage is applied between the cell array and the module frame. In repeating intervals the water film is removed and characteristic electric parameters are measured, in particular the current-voltage characteristics under illumination or the shunt resistance. Sometimes also a conductive coating is applied to the front side of the modules. In such a case, it must be removed in a time-consuming manner, before the current-voltage-characteristics under illumination are measured.
These test methods relate to completed photovoltaic modules, wherein the conductivity on the front side is increased by means of a water film or a conductive coating. On the other hand, a reliable test of individual solar cells does not exist, in particular a rapid test. Thus, there is no possibility to evaluate individual solar cells before being mounted into a photovoltaic module and possibly to sort out damaged cells. Also, no rapid test for photovoltaic modules exists, which avoids the use of water or high humidity or of a conductive coating.
EP 1 274 760 B1 and US 2007/0246094 A1 disclose electrodes of plastic material for solar cells that are either fixedly connected to the semiconductor surface by layer-deposition-techniques or that are present in the form of measuring tip electrodes that can be applied only locally onto a surface.
From EP 1 024 369 A1 detachable large area electrodes are known in the form of electrolytes, but this represents a different approach.